Mercury 1975 7.5hp flywheel backlash

[cheboygan]

I have a 1975 Mercury outboard, 7.5 HP motor recently acquired. Have not yet run same.There is a 7 degree "backlash" in the flywheel movement. Is this normal, or does it indicate excessive needle bearing wear at the bearing journal and/or piston wristpin? ie. Is a teardown and rebuild called for? efwhite

 

[rodbolt]

Re: Mercury 1975 7.5hp flywheel backlash

how are you checking it?<br /> and by backlash what are you checking?<br /> a quick way to check rod/wristpin bearing wear is turn the engine in its normal rotation until the piston passes TDC and starts down about 1/4 of its stroke. then hold the flywheel and push on the piston top with a long phillips. no motion is good. any downward motion while the flywheel does not move means teardown.

 

[cheboygan]

Re: Mercury 1975 7.5hp flywheel backlash

By backlash, I mean the ability to rotate the flywheel freely back and forth 7 degree before coming to a starting point, as determined by forcefull pressure, of moving the pistons one way or the other. Degree is measured by a vertical line on the flywheel moving between degree marks banded on the rope starter cover housing. efwhite

 

[rodbolt]

Re: Mercury 1975 7.5hp flywheel backlash

there is a few degrees of crankshaft rotation that is considered "dead". its where the piston is at the top of the crank stroke, the backlash can really only be measued with a dial indicator and a known good degree mark. it will happen again at the bottom of the stroke. as far as I know there is no specification for it but somewhere along the linear motion of the piston you must slow it down,stop it and accelerate it the other way only to repeat it again at the end of that stroke.

 

[emckelvy]

Re: Mercury 1975 7.5hp flywheel backlash

You're probably feeling tension from the impeller blades trying to go backwards.<br /><br />Not recommended to rotate the flywheel other than in the normal direction of rotation of the motor (clockwise), as you're going to bust impeller blades if you continue to do so.<br /><br />Impeller blades take a 'set' after running for so long and do not take kindly to being reversed!<br /><br />Seeing as how you've not run it and it's probably not known the last time the impeller was replaced, I'd recommend doing that before you even attempt to run it. If you break an impeller blade, it's possible for the broken chunks to be forced up the water tube, causing even more grief.<br /><br />You can get Sierra replacement parts from a number of places such as NAPA or www.boatfix.com or for factory parts, go to the Merc official site, www.mercurypartsexpress.com.<br /><br />HTH.......ed